1. Field of the Invention
The present invention relates to an optical recording medium. More particularly, the present invention relates to an optical recording medium for use in devices, such as an optical disk, optical card, optical tape and so forth, wherein information is recorded by irradiation of an energy beam such as a laser beam or an electron beam.
In particular, the present invention relates to an optical recording medium which enables information to be recorded at a high speed and a high density, has a high reliability and is excellent in the quality of read-out signals.
2. Description of the Prior Art
Various materials, such as In-Se alloy system, Te sub-oxide system, Sb-Te alloy system and Te-Ge alloy system thin films, have been proposed as an optical recording medium which takes advantage of the difference in optical properties accompanying the phase transition from crystalline state to amorphous state and vice versa. In this connection, one of the methods for evaluating an optical recording medium preferable from the viewpoint of compatibility with a drive system is that standards of the write once type and rewritable type recording media which are in the process of standardization by ISO (International Organization for Standardization) are used as a criterion for evaluating the performance of a recording medium and the performance is judged based on whether the media is well matched to the standards. However, for example, regarding the write once type recording medium, it is not easy to develop a medium material fulfilling the standards, i.e., having a good balance of characteristics in respect of a recording density, a carrier to noise ratio (hereinafter referred to as "CNR") representing a signal quality and a recording sensitivity as well as having excellent life of media.
For this reason, many studies have been made on an improvement in the characteristics of the recording layer of the above-described material system. In particular, a Te-Ge alloy material is advantageous in that the change of reflectance (signal margin) between before and after the phase transition is relatively large and well-known thin film forming techniques, such as vacuum evaporation and sputtering, can be utilized. Therefore, several ideas on an improvement in the characteristics through addition of various elements to the Te-Ge alloy to form three- or four-component alloy materials have been proposed.
Specifically, compositions comprising Te-Ge and Bi or Bi.sub.2 Te.sub.3 are known as a three-component alloy system material (see Japanese Patent Application Kokai Publication Nos. 62-209741, 63-225935 and 1-165048), and Japanese Patent Application Kokai Publication No. 63-155442 discloses a composition comprising Te as a major component and Ge and Bi added thereto. Although in a recording media comprising the above-described compositions as a recording layer, a crystallized mark can be formed at a practical laser power, such recording media had drawbacks that no sufficient CNR can be obtained in a high-density recording and/or a high-speed recording and/or the transition temperature of crystallization is lowered in a composition having a high Bi content to bring about a lowering in the thermal stability.
Four-component alloy system recording layers are disclosed in Japanese Patent Application Kokai Publication Nos. 62-152786, 61-152487, 62-145547 and 64-89046.
The Japanese Patent Application Kokai Publication No. 62-152786 describes that all kinds of elements can be added to Te-Ge alloy and the compositions exhibit good properties. However, only addition of Ti and Co to Te-Ge alloy is disclosed in the working examples, and neither specific studies nor specific disclosure is made on practical recording characteristics such as CNR or media noise. Evaluation of the recording media disclosed in the Japanese Patent Application Kokai Publication No. 62-152786 according to the ISO standards had revealed that they are insufficient in the CNR under high-density recording conditions and/or bring about a lowering in the signal quality due to an increase in the noise, etc. Therefore, their characteristics are unsatisfactory from the practical point of view. Besides the above-described addition of Ti and/or Co described in the working examples, disclosure on other elements which may be added to Te-Ge is only that part of Ti, Co or Ge may be replaced with, e.g., halogen elements, alkali metal elements, Ti, Pb, Sb, Au, Sn, Bi, In and Ga.
In particular, this publication does not clearly disclose that Bi and Ga makes various practical characteristics of media such as CNR and/or noise characteristics better, not to mention useful and positive finding on the function and effect of a four-component alloy system having a limited composition. For example, recording films comprising Te-Ge and separately added thereto Bi and Ga have drawbacks such as difficulty of obtaining a sufficient CNR value in high-density recording and/or lowering in the reliability of recorded information due to a lowering in the transition temperature of crystallization. Therefore, these materials are not regarded as practical.
The Japanese Patent Application Kokai Publication No. 61-152487 proposes addition of Group III, IV and V elements on the periodic table having atomic number 31 or higher to Te-Ge alloy. The essential feature of this proposal resides in the addition of one of the above-described elements to Te-Ge alloy. Specifically, only one composition comprising Te-Ge and Sb added thereto is disclosed as a working example, reference is made to only the effect of addition of Bi or In, and this publication does not disclose an improvement in various practical properties of a recording medium, such as CNR or noise characteristics, through addition of Group III, IV and V elements, not to mention the function and effect of preparation of a four-component alloy system having a limited composition through simultaneous addition of Bi and Ga to Te-Ge alloy as with the above-described Japanese Patent Application Kokai Publication No. 62-152786.
The Japanese Patent Application Kokai Publication No. 62-145547 describes that small amounts of Te and Ge are added to a composition mainly composed of a Ga-Bi alloy. However, only a recording layer of In-Bi alloy is described in the working example. This working example refers to an improvement in the sensitivity through an increase in the optical absorbance by making use of a particular material, such as ZnS, as the protective layer, and this publication is silent on whether or not the recording layer per se can realize practical and excellent recording characteristics.
Finally, the Japanese Patent Application Kokai Publication No. 64-89046 discloses a recording layer prepared by adding Ge and Bi to an alloy mainly composed of Ga-Te alloy and describes that when the total amount of Ge and Bi exceeds 20%, the structure of Ga-Te alloy is damaged, so that desired properties cannot be attained. Further, the Japanese Patent Application Kokai Publication No. 64-89046 has a working example on addition of Bi to Ga-Te alloy but has no clear description on a four-component alloy system containing Ge as well, i.e., disclose neither useful nor positive finding on the function and effect of an alloy system comprising four elements of Te, Ge, Ga and Bi and having a limited composition.
Further, the compositions disclosed in the above-described Japanese Patent Application Kokai Publication Nos. 62-145547 and 64-89046 have drawbacks such as insufficient crystallization, lowering in the crystallization speed and/or lowering in the transition temperature as well as small amplitude of read-out signal and/or low CNR due to insufficient change of reflectance of media.